\Kinetic and Isotherm Modelling of adsorption of Cr3+
metal ions from Tannery wastewater on to unmodified and acid-modified Arabica
coffee husks biosorbents
Charles
Owino Oloo*, Nthiga E. W, Douglas Onyancha, Gerald K. Muthakia
Department of Chemistry, School of Science, Dedan Kimathi
University of Technology,
Private Bag, 10143 - Dedan Kimathi, Nyeri-Kenya.
*Corresponding Author E-mail: owinocharles90@gmail.com
The kinetics and isotherm modeling of adsorption of Cr3+
ions onto unmodified coffee husk (UCH) and acid modified coffee husk
(MCH) biosorbent were successfully studied in batch experiments. Equilibrium
data were analyzed by Langmuir and Freundlich isotherm models. Equilibrium data
best fitted to Langmuir isotherm. The Langmuir adsorption capacity was higher
in MCH (6.64 mg/g) compared to the UCH (3.85 mg/g). The adsorption constant,
b, was less than one in the two adsorption processes suggesting a high
affinity of both the UCH and MCH adsorbent for Cr3+ ions. The
experimental data were fitted well with pseudo second order kinetics with best
fitness for both the UCH and MCH with regression coefficient (R2) of
0.9792 and 0.9956, respectively. The findings revealed that the unmodified and
modified coffee husks have potential in removal of Cr3+ from tannery
wastewater
INTRODUCTION:
Tanneries worldwide produce approximately 40 million
litres of wastewater that contain chromium every year1. Most industries
discharge tannery wastewater into sewerage system without proper treatment or
in its raw form causing serious environmental impact. There is public concern
related to hexavalent compounds owing to their toxic effects on humans, plants,
microorganisms and animals. The risks on human health is dependent on the
amount, exposure level and duration. Exposure to chromium can damage the skin,
eyes, blood, respiratory and immune system2.
Treatment of tannery wastewater in most tanneries uses
physic-chemical methods that include reverse osmosis, precipitation, membrane
processes and adsorption3,12. Chemical precipitation is the most
considered effective technique used to separate heavy metals from industrial
waste4.
Most commonly used precipitating agents include:
calcium hydroxide, magnesium oxide or calcium magnesium carbonate and sodium
hydroxide. Precipitated chromium hydroxide can be easily recycled for tanning
process by re-dissolving using an acid.
Adsorption has been universally used in removal of
pollutants because of the low cost, low consumption of the chemicals and
reagents, ease in handling, as well as ability to recover value added
components through desorption and regeneration of adsorbent5. The
current focus in the field of chemistry primarily revolves around assessing the
potential of readily available, sustainable, and cost-effective materials as
effective adsorbents to address the pressing issue of heavy metal pollution.
This study was initiated in response to this growing trend, and it seeks to
adapt Arabica coffee husk as an adsorbent for the removal of Cr3+
ions. The uniqueness of this research lies in its approach to adsorbent
synthesis and the comprehensive evaluation of adsorption models.
MATERIALS AND METHODS:
Materials:
The chemicals employed in this investigation were of
analytical quality, exhibiting a purity exceeding 98.9%. Sodium acetate,
sulfuric acid, hydrochloric acid, and chromium chloride hexahydrate were
procured from Sigma Aldrich (Kobian, Nairobi). The Arabica coffee husks
utilized originated from the coffee farm at Dedan Kimathi University in Kenya.
Adsorbent:
Coffee husk biomass was extensively washed in
double-distilled water. After washing and drying the samples were oven-dried at
105°C until there was no longer any weight change, the samples were sieved
through a fine sieve. Two sections of the samples were made; one piece
underwent acid modification (MCH). The other was labelled "unmodified
coffee husk" (UCH) and used in its original form. The chemical alteration
was carried out in accordance with Owino et al.'s6,13 method.
Adsorption experiments:
Adsorption experiments were performed in 250mL glass
conical flasks at ambient temperature, with agitation provided by a mechanical
shaker operating at 120 revolutions per minute. The adsorption parameters that
influenced the process (such as pH, the starting concentration of Cr3+
ions, the biomass dose, and contact time) had already been determined, Owino et
al.,6. The optimized parameters were, biomass dosage 3g, pH 4.5,
metal ion concentration 50mg/L, and time 15min and 25 min for the MCH and UCH,
respectively.
Adsorption isotherm and effect of metal ion
concentration:
In 100mL Erlenmeyer flasks, batch sorption isotherm
studies were conducted. The amount of chromium ions absorbed (qeq) in relation
to metal concentration was calculated as follows: For the unmodified and
acid-modified coffee husks, 3g of finely ground coffee husks was combined with
10mL of buffer at pH 4.5 and 10mL of chromium (III) solution in the range, 10 -
100mg/L. For two hours, each mixture was stirred. After centrifuging the
mixture, the amount of chromium (III) ions (Ceq) in the supernatant solution
that was in equilibrium with the coffee husks was calculated using AAS.
According to Onyancha et al.7, the same method was used to determine
the concentration in tannery waste.
Adsorption kinetics:
Approximately 3g of coffee husks were combined with
10mL of a 10mg/L Cr(III) solution and 10mL of acetate buffer at pH 4.5 to study
the rate of chromium adsorption by both unmodified and acid-modified coffee
husks. The mixture was equilibrated in the conical flask. A membrane-equipped
syringe was used to draw 2mL of the solution at specified intervals of 10min,
and the concentration of chromium was then calculated6.
Adsorption kinetic models:
The study employed the Pseudo-first-order and
Pseudo-second-order adsorption kinetics models, chosen for their convenience in
describing solid-solid interactions and the rate of adsorption behaviour. Two
kinetic models, pseudo-first order and pseudo-second order, which are depicted
in equations (1) and (2), respectively, were chosen to correlate with the
experimental data based on their stability towards the solid-liquid adsorption
characteristics. The results of the kinetics experiments were analysed to predict
the constants of adsorption using these models8.
(1)
(2)
RESULTS AND DISCUSSION:
Adsorption isotherms:
To evaluate the adsorption process and determine the
maximum adsorption capacity of the adsorbent, the adsorption data were fitted
to the Langmuir and Freundlich equations. The Langmuir equation is based on the
assumption that the maximum sorption corresponds to a saturated monolayer of
sorbate molecules on the sorbent surface. In order to assess the adsorption process and ascertain
the highest adsorption capacity of the adsorbent, the adsorption data were
subjected to fitting with the Langmuir and Freundlich equations. The Langmuir
equation operates under the premise that the maximum sorption is associated
with a saturated monolayer of sorbate molecules on the surface of the sorbent.
Based on the results presented in Figures 1 and 2, it
can be concluded that monolayer adsorption is the predominant type of
biosorption for metal ions onto both untreated coffee husk (UCH) and
acid-modified coffee husk (MCH). The Langmuir isotherm model provides a better
fit to the data than the Freundlich isotherm model, as evidenced by the higher
regression (R2) values. Specifically, the R2 values for
UCH and MCH with the Langmuir model were 0.9908 and 0.9912, respectively. In
contrast, the R2 values obtained using the Freundlich isotherm for
UCH and MCH were 0.9755 and 0.9777, respectively. However, the lower R2
values associated with the Freundlich isotherm model indicate that it
inadequately describes the relationship between the amount of adsorbed metal
ions and their equilibrium concentration in the solution
A B
Fig 1: Langmuir isotherm (a) and Freundlich isotherm
(b) plots for adsorption of Cr3+ onto UCH
Fig 2: Langmuir isotherm (a) and Freundlich isotherm
(b) plots for the adsorption of Cr3+ onto modified coffee husk
Freundlich and Langmuir's models were used to analyse
the Cr3+ ions adsorption capacity by the adsorbents. The findings
are illustrated in table 1.
The research results in table 1 show a high
correlation between the qe (experimental) and qe (calculated). From the
calculated values of R2 (>0.991), the results fitted well on
Langmuir isotherm model (R2>0.990) as compared to Freundlich
isotherm. This means that the adsorption of Cr3+ ions on unmodified
and modified coffee husk adsorbents can be described by chemisorption and the
adsorption is on a monolayer9,14. The values of the adsorption
capacities qe for the modified adsorbent (6.64mg/g) were higher than those of
the unmodified adsorbent (3.85mg/g). The b value being less than one in
the two adsorption processes suggests a high affinity of both the modified and
unmodified adsorbent for Cr3+ ions.
Adsorption kinetics:
The adsorption kinetics of Cr3+ ions onto
UCH and MCH were investigated using two kinetic models, namely the
pseudo-first-order and pseudo-second-order models. The findings are reported in
table 2:
Table 1: Langmuir and Freundlich constants for Cr3+
adsorption on UCH and MCH
|
Langmuir
|
Freundlich
|
|
Ads.
|
qe, exp.
(mg/g)
|
qe, cal (mg/g)
|
b
(mg/g)
|
R2
|
KF
(mg/g)
|
1/n
|
R2
|
Best model
|
|
UCH
|
3.85
|
3.63
|
2.18 × 10-2
|
0.9908
|
0.15
|
0.6146
|
0.9755
|
Langmuir
|
|
MCH
|
6.64
|
6.50
|
3.85 ×10-2
|
0.9912
|
0.37
|
0.4894
|
0.9772
|
Langmuir
|
Table 2: Pseudo-first-order and Pseudo-second-order constants
for Cr3+ using UCH and MCH
|
Ads.
|
Pseudo-first-order
|
Pseudo-second-order
|
|
qe, exp
(mg/g)
|
qe, cal
(mg/g)
|
k1
(mg/g/min)
|
R2
|
qe, cal
|
k2
(mg/g/min)
|
R2
|
Best model
|
|
UCH
|
0.8898
|
0.1677
|
1.7 × 10-2
|
0.9016
|
0.7813
|
1.05
|
0.9792
|
Pseudo-second order
|
|
MCH
|
1.2488
|
2.7946
|
5.24
|
0.9563
|
1.1186
|
10.6
|
0.9956
|
Pseudo-second order
|
Fig 3. Pseudo 1st order plot of Cr3+
ion adsorption onto UCH (A) Pseudo 1st order plot of Cr3+
ion adsorption onto MCH (B)
Fig 4: Pseudo 2nd order plot of Cr3+
ions onto UCH (A) Pseudo 2nd order plot of Cr3+ onto MCH
(B)
The results in table 2 show poor R2 values
in the pseudo-first-order kinetics and small calculated 
values and the experimental values. The adsorption did not fit the
pseudo-first-order of kinetics because the values were very far from each
other. With an R2 value greater than 0.9, the pseudo-second-order
was a good fit for explaining the kinetics. Furthermore, the experimental and
calculated values for Cr3+ ions on
unmodified and modified adsorbents were nearly identical, implying a
chemisorption adsorption mechanism3 attributed to valence forces
through sharing or ion exchange of electrons between the electron-rich surface
sites and electron-deficient metal ions10.
In modified form, the adsorption rate was higher
compared to the unmodified one. The modified adsorbent registered a 10.6mg/g/min
rate compared to 1.05mg/g/min in the unmodified one. The capacity to adsorb for
the value of
, cal was lower in the unmodified form than
in the modified one. MCH adsorbent showed the greatest value of 1.1186mg/g
compared to 0.7813mg/g in the unmodified one. This translates to it that the
chromium (III) ions adsorption takes a shorter time to complete the process and
also the chemical modification of the material improves the capacity to adsorb.
After modifying the coffee husk using sulphuric acid, the surface structure and
charge of the adsorbent changed11.
CONCLUSIONS:
At room temperature and 120 rpm, the adsorption
capacity, adsorption isotherm, and kinetic behaviour of unmodified and modified
coffee husks in Cr3+ adsorption were studied. Compared to the
original coffee husk, the modified coffee husk had a greater adsorption
capacity. For this study, the selection of adsorption isotherm models included
Freundlich and Langmuir, while the kinetic models applied were the pseudo-first
order and pseudo-second order. The research data demonstrated that the
pseudo-second order adsorption kinetic model provided the best fit, as
indicated by its high regression value (R2).
ACKNOWLEDGMENTS:
Dedan Kimathi University (DeKUT) provided support for
this study, which the authors would like to acknowledge. Kenyatta University
was also credited for the FT-IR and AAS studies.
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